Demarcating Old Rocks

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Science  21 Jan 2000:
Vol. 287, Issue 5452, pp. 393
DOI: 10.1126/science.287.5452.393g

Crust that is greater than 3.0 billion years old is distributed in the cores of cratons (stable areas of Earth's crust); these can be found in the interiors of several continents. Identifying and correlating these cores provides clues to the rate of formation of continental crust through time and to Earth's geochemical evolution, and also reveals whether some form of plate tectonics operated early in Earth's history.

Qiu et al. have identified 3.2 billion year old rocks in the Yangtze craton located in south China. Rocks of this age were thought to be restricted to the North China craton, which was joined with the Yangtze craton only 220 million years ago along a major orogenic belt (the Qinling-Dabie-Sulu suture). Separately, Böhm et al. have identified crust at least 3.5 billion years old (and perhaps from 3.8 billion years ago) along the ancient margin of the Superior Province located in Manitoba, Canada (the large craton core in eastern and central Canada). Both studies used uranium-lead dating of zircons to date directly the formation of igneous rocks and samarium-neodynium geochemical analysis of whole rocks to estimate the age of extraction of crust from the mantle.—BH

Geology28, 11 (2000); Geology28, 75 (2000).

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